A conventional known heating apparatus switches turning-on/off of the current feeding from an AC power source to a heater of a heating apparatus to heat the heater at a predetermined current-feed ratio. As a conventional technique of restraining occurrence of high-frequency wave, in other word, harmonic current in current-feed of the heating apparatus, for example, a technique is known which turns on the current-feed by 100% when the heater temperature is less than a lower limit value, turns off the current-feed when the heater temperature is higher than an upper limit value, and a sine-wave alternating current (AC) is periodically turned on/off in synchronization with zero cross of a sine-wave AC when the heater temperature falls between the upper limit value and the lower limit value.
According to the conventional technique, high frequency wave occurring at turning-on/off of the sine-wave alternating current can be reduced. However, a standard value of a harmonic current in heaters has been recently become strict and therefore, in heating control of the heaters, a technique of further restraining the harmonic current has been demanded. For example, a method of further restraining the harmonic current by forcibly inserting a period of current-feed ratio of 100% or the current-feed ratio of 0%, during which the harmonic current hardly occurs, in current-feed control of the heaters can be considered. However, according to this method, a balance between restraint of the harmonic current and the control accuracy of the heating temperature of the heating apparatus is important. Thus, there has been a demand for keeping the control accuracy of the heating temperature while improving the effect of restraining the harmonic current.
The present invention provides a technique of keeping the predetermined control accuracy of the heating temperature while improving the effect of restraining the harmonic current in heating control of the heater.